In the course of collecting data for other purposes, it was found that both male and female rats living in activity wheels from two weeks post-weaning maintain the same body weights as sedentary rats even though they turned a wheel of 1 m circumference a mean of over 9600 (females) or 8200 (males) revolutions daily after reaching sexual maturity and ate 4.0 g (females) or 3.5 g (males) of lab chow daily more than sedentary rats. The great difference in weight control by these rats, especially the males, from rats forced to excerise and from older rats introduced to activity wheels, suggested exploration of the hypotheses that the enforced inactivity in the typical housing of laboratory rat and the enforced character of forced exercise may contribute as much or more to the typical control of the body weights of laboratory animals by exercise as the fact of exercise per se. To these ends, the present proposal is (1) to extend the period during which rats are reared in activity wheels and also incorporate carcass analyses for fat, (2) to rear rats on the Mickelsen high-fat diet and in activity wheels to examine interactions of adiposity and activity apart from age or an initial sedentary existence, and (3) to introduce to forced exercise on a treadmill both rats reared in activity wheels and sedentary rats.